The advent of graphene (K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, “Electric field Effect in Atomically Thin Carbon Films” Science, Vol. 306, No. 5696, pp. 666-669, 2004.) and subsequent discovery of its multitude of superior properties, has led to the identification of many other two-dimensional crystals through exfoliation of suitable layered compounds. These materials are all molecular and are typically compounds formed from 2, 3, 4 or 5 different elements. Compounds which have been isolated as single- or few layer platelets or crystals include hexagonal boron nitride, NbSe2, bismuth strontium calcium copper oxide (BSCCO) and MoS2. These single or few layer platelets or crystals are stable and can exhibit complementary electronic properties to graphene, such as being insulators, semiconductors and superconductors.
The large variety of 2D crystals isolated in the recent years offers a rich platform for the creation of heterostructures which combine several of these compounds in one stack. Since, collectively, this class of compounds covers a very broad range of properties, the obtained heterostructures can be tuned to focus on particular phenomena, or be used for specific applications (or even to perform multiple functions).
Thus, inorganic single or few layer compounds can be used either alone or in combination with other such materials and/or with graphene to form ultrathin electronic devices with astonishing properties. BN and MoS2 have been used in conjunction with graphene to form quantum tunnelling transistor heterostructures (WO2012/127245) while MoS2 and WS2 have been used in conjunction with graphene to form photovoltaic heterostructures (WO2013/140181).
Still, up to now heterostructures have predominantly been produced by micromechanical cleavage of three-dimensional layered crystals with subsequent dry transfer of each crystal layer. While this technique allows one to achieve extremely high quality heterostructures. This technique is labour intensive and time consuming and it cannot be applied to the production of such heterostructures on a large scale. Ultimately, this method is likely to be unsuitable for mass-production, and is unlikely to be able to satisfy demand for these materials for use in real-life applications.
Thin films of inorganic compounds can be formed from the deposition of dispersions of flakes of such compounds. Liquid-phase exfoliation has been employed as a scalable approach for production of two-dimensional crystals in dispersed form. This method is based on exfoliation of their bulk counterparts via chemical wet dispersion followed by ultra-sonication. Whilst this technique offers many advantages for cost reduction, scalability and compatibility with any substrate, including cheap and flexible substrates, existing methods suffer the disadvantage of requiring media which are not environmentally friendly. Currently this is mostly based on the use of organic solvents such as N-Methylpyrrolidone (NMP) and N,N-dimethylformamide (DMF), which are chosen because of their compatibility with the resulting dispersed phase. These solvents are invariably toxic, expensive and characterized by high boiling points. While these solvents can produce a high yield of single-layer graphene in suspension, they are also less efficient in exfoliating graphene analogues.
WO2012/028724 describes a method of exfoliating MoS2 with water/surfactant (e.g. sodium cholate) mixtures to provide suspensions which are suitable for forming thin films made up of individual flakes of MoS2. The concentrations of the inorganic compounds in the resultant suspensions are low (see also Smith et al, Adv. Mat. 2011, 23, 3944). This may be because these solvent/surfactant systems have only low effectiveness in supporting the dispersed phase.
Zhang et al (Chem. Comm., 2013, 49, 4845-4847) have shown that a hyaluronan backbone substituted with pyrenes can be used to generate a dispersion of hyaluronan supported two dimensional materials, such as BN and MoS2, suitable for use in delivering the two dimensional materials to biological systems. It is believed that the hyaluronan backbone has a role both in achieving exfoliation and in increasing the stability of the resultant suspensions. Less than 20% of the available carboxylic acids in the hyaluronan support are functionalised with pyrenes. The loading of the two-dimensional material onto the hyaluronan backbone is small. It seems unlikely that the dispersions generated from this method would be suitable for the formation of thin films with certain properties which because the large amounts of the polysaccharide hyaluronan substrate would be present in the resultant deposited thin films. It seems likely that this would be detrimental to the integrity of the deposited thin films.
Pyrene sulfonic acids have been shown to be effective at exfoliating graphene to form dispersions in aqueous media (Yang et al; Carbon, 53, 2013, 357-365; Schlierf et al; Nanoscale, 2013, 5, 4205-4216). The efficiency of the process depends on a range of factors: the thermodynamics of exfoliation; the presence of local energy minima influencing the kinetics of the process; and solvent-molecule, solvent-graphene and graphene-molecule competitive interactions. It is understood that the unique electron distribution in graphene contributes to the success of this exfoliation process. Pyrene molecules associate with graphene through π-π interactions. Such π-π interactions would be expected to be considerably weaker in strength between polyaromatic compounds such as pyrene and inorganic layered compounds. These compounds do not enjoy the same uniform electron distribution that is present in graphene. Furthermore, the bonding between layers of inorganic layered compounds is very different to that between graphene layers. For example, in the case of h—BN, the bonding between neighbouring BN layers is formed by so called ‘lip-lip’ interactions, which would be expected to be stronger than the weak Van der Waals forces operating between graphene layers.
It is an aim of certain embodiments of the invention to provide a method for exfoliating layered inorganic compounds to form aqueous dispersions of single and few-layered inorganic compounds in high yields. In is an aim of certain embodiments to provide methods of forming said dispersions in higher yields than those made by prior art methods.
It is an aim of certain embodiments of the invention to provide a method for exfoliating layered inorganic compounds to form aqueous dispersions of single and few-layered inorganic materials in high concentrations. In is an aim of certain embodiments to provide methods of forming said dispersions in higher concentrations than those made by prior art methods.
It is an aim of certain embodiments of the invention to provide a method for exfoliating layered inorganic compounds to form stable aqueous dispersions of single and few-layered inorganic materials. It is an aim of certain embodiments of the invention to provide a method for exfoliating layered inorganic compounds to form aqueous dispersions of single and few-layered inorganic materials which are more stable than those made by prior art methods.
Thus far, no method has been developed for the preparation of aqueous dispersions of single and few-layered layered inorganic compounds at a sufficiently high concentration for an effective thin film to be printed in a single coat or in only a few coats. It has also not been shown that thin films can be formed of a high enough quality from dispersions in such media to exhibit properties similar to those of the heterostructure based devices formed from single crystals of inorganic single- or few-layer compounds. It is thus an aim of certain embodiments of this invention to provide a method for exfoliating layered inorganic compounds to form an aqueous dispersion suitable for forming thin films. Another aim of certain embodiments of the invention is to provide high quality dispersions which can be deposited in the form of thin films without leaving undesirable residues and/or which do not require significant further treatments to ensure the integrity of the deposited film.